Methods and apparatus for test gas leak detection

ABSTRACT

A leak detector includes a leak detector inlet to receive a test gas, a vacuum pump coupled to the leak detector inlet, a test gas sensing unit connected through a passage to the leak detector inlet, to sense the test gas, a membrane that is permeable to the test gas, the membrane disposed in the passage between the leak detector inlet and the test gas sensing unit, and a control mechanism to substantially terminate transmission of the test gas through the membrane in a time of 500 milliseconds or less. In other embodiments, the control mechanism substantially terminates transmission of the test gas through the membrane in a time of 100 milliseconds or less.

RELATED APPLICATION

This application is related to the application entitled “Test Gas LeakDetection Using a Composite Membrane” by Anthony Liepert, whichapplication is being filed on the same date as the present applicationand is assigned to the assignee of the present application.

FIELD OF THE INVENTION

This invention relates to detection of leaks in sealed articles and,more particularly, to methods and apparatus for test gas leak detectionwhich utilize a membrane to pass the test gas to a test gas sensingunit.

BACKGROUND OF THE INVENTION

Helium mass spectrometer leak detection is a well-known leak detectiontechnique. Helium is used as a test gas, which passes through thesmallest of leaks in a sealed test piece. After passing through a leak,a test sample containing helium is drawn into a leak detectioninstrument and is measured. In the instrument, a mass spectrometer tubedetects and measures the helium. The input test sample is ionized andmass analyzed by the spectrometer tube in order to separate the heliumcomponent. In one approach, a test piece is pressurized with helium. Asniffer probe connected to the test port of the leak detector is movedaround the exterior of the test piece. Helium passes through leaks inthe test piece, is drawn into the probe and is measured by the leakdetector. In another approach, the interior of the test piece is coupledto the test port of the leak detector and is evacuated. Helium issprayed onto the exterior of the test piece, is drawn inside through aleak and is measured by the leak detector.

One of the difficulties associated with helium mass spectrometer leakdetection is that the inlet of the mass spectrometer tube must bemaintained at a relatively low pressure, typically 2×10⁻⁴ Torr. In aso-called conventional leak detector, the test port, which is connectedto the test piece or to the sniffer probe, must be maintained atrelatively low pressure. Thus, the vacuum pumping cycle is relativelylong. Furthermore, in the testing of leaky or large volume parts, it maybe difficult or impossible to reach the required pressure level. If therequired pressure level can be reached, the pumping cycle is lengthy.

Techniques have been proposed in prior art to overcome this difficulty.A counterflow leak detector is disclosed in U.S. Pat. No. 3,690,151,issued Sep. 12, 1972 to Briggs, utilizes a technique of a reverse flowof helium through a diffusion pump to the mass spectrometer. The leakdetector test port can be operated at the pressure of the diffusion pumpforeline. A similar approach uses reverse flow of helium through aturbomolecular pump. U.S. Pat. No. 4,735,084, issued Apr. 5, 1988, toFruzzetti, discloses a technique for gross leak detection wherein thetest gas is passed in reverse direction through one or two stages of amechanical vacuum pump. These techniques have permitted the test portpressure to be higher than for conventional leak detectors. Nonetheless,reaching the higher test port pressure can be difficult when testinglarge volumes, dirty parts, or parts with large leaks.

French Patent No. 1,181,312, published on Jun. 15, 1959, discloses ahelium leak detector that uses a heated silica membrane to selectivelypass helium and hydrogen. A getter is provided to capture the hydrogen,and an ionization gauge is used to measure the helium pressure. EuropeanPatent Application No. 0352371, published Jan. 31, 1990, discloses ahelium leak detector including an ion pump connected to a probe in theform of a silica glass capillary tube. The silica glass tube is heatedto a temperature between 300° C. (degrees centigrade) and 900° C. andthereby becomes permeable to helium. U.S. Pat. No. 5,325,708, issuedJul. 5, 1994, to DeSimon, discloses a helium detecting unit using aquartz capillary membrane, a filament for heating the membrane and anion pump. U.S. Pat. No. 5,661,229, issued Aug. 26, 1997, to Bohm et al.,discloses a leak detector with a polymer or heated quartz window forselectively passing helium to a gas-consuming vacuum gauge.

Leak detection techniques, which utilize a permeable membrane permit thetest gas sensor to operate at a different pressure from the leakdetector inlet. However, prior art membranes have had low permeance atroom temperature and have typically required heating to a hightemperature to increase permeance. The heated membrane requires acontrolled heating source, thereby increasing the cost and complexity ofthe unit. In addition, the permeance of prior art membranes has had arelatively large temperature coefficient. Thus, the accuracy of leakdetection depends in part on the accuracy with which the temperature ofthe membrane is controlled.

Leak detection techniques which utilize a permeable membrane are alsovulnerable to too much helium reaching the ionization gauge, ion pump,or gas-consuming vacuum gauge. If a large leak is encountered, theresulting helium partial pressure outside the membrane drives a largenumber of helium atoms into the sensor. Once there, these sensors cannotrapidly bury all these helium atoms. This can render the leak detectorinoperable, or with reduced sensitivity, for several minutes.

Accordingly, there is a need for improved methods and apparatus for leakdetection.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a leak detector isprovided. The leak detector comprises a leak detector inlet to receive atest gas, a vacuum pump coupled to the leak detector inlet, a test gassensing unit connected through a passage to the leak detector inlet, tosense the test gas, a membrane that is permeable to the test gas, themembrane being disposed in the passage between the leak detector inletand the test gas sensing unit, and a control mechanism to substantiallyterminate transmission of the test gas through the membrane in a time of500 milliseconds or less.

In other embodiments, the control mechanism substantially terminatestransmission of the test gas through the membrane in a time of 100milliseconds or less. In further embodiments, the control mechanismsubstantially terminates transmission of the test gas through themembrane in a time of 25 milliseconds or less.

In one embodiment, the control mechanism comprises a first valve coupledbetween the leak detector inlet and the membrane, a trapped volume beingdefined between the first valve and the membrane, a second valve coupledto the trapped volume, and a controller to open the first valve andclose the second valve in a first mode and to open the second valve andclose the first valve upon switching to a second mode, wherein thevacuum pump comprises a dual inlet vacuum pump having a first inletcoupled to the leak detector inlet and a second inlet coupled throughthe second valve to the trapped volume.

In another embodiment, the control mechanism comprises a valve disposedin the passage between the leak detector inlet and the test gas sensingunit, the valve including a housing and a valve closure member movablebetween a closed position in contact with the membrane and an openposition retracted from the membrane.

According to a second aspect of the invention, a method is provided forleak detection. The method comprises passing a test gas through amembrane disposed in a passage between a leak detector inlet and a testgas sensing unit, and sensing the test gas with the test gas sensingunit, and substantially terminating transmission of the test gas throughthe membrane in a time of 500 milliseconds or less.

In other embodiments, the method includes substantially terminatingtransmission of the test gas through the membrane in a time of 100milliseconds or less. In further embodiments, the method includessubstantially terminating transmission of the test gas through themembrane in a time of 25 milliseconds or less.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand which:

FIG. 1 is a simplified block diagram of a first example of a leakdetector;

FIG. 2 is a partial cross-sectional view of an example of a valve whichincorporates a membrane;

FIG. 3 is a simplified block diagram of a second example of a leakdetector;

FIG. 4 is a graph of permeance as a function of temperature of a firstexample of a composite membrane;

FIG. 5 is a graph of permeance as a function of temperature of a secondexample of a composite membrane; and

FIG. 6 is a schematic diagram of an example of a reference leak.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates in general to leak detectors that use one or morelight gases as the test gas. Light gases include helium, hydrogen, neon,and isotopes thereof. Leak detection methods and apparatus include amembrane disposed between an inlet and a test gas sensing unit. Themembrane is permeable to the light gases and selectively blocks heaviergases, i.e., all gases heavier than neon. Examples of test gas sensingunits include an ion pump, an ionization gauge, a mass spectrometer, amagnetic sector mass spectrometer, a quadrupole mass spectrometer, apenning cell, a gas-consuming vacuum gauge, and a charge coupled device.From the output of the test gas sensing unit as a function of time andthe valve modulation history, a near instantaneous test gasconcentration, and hence leak rate, can be computed.

An issue in leak detectors which utilize a membrane that passes the testgas and selectively blocks heavier gases is how to terminatetransmission of the test gas through the membrane rapidly in response,for example, to switching the leak detector from a test mode to anon-test mode. In the test mode, a connection is established between theinlet of the leak detector and the membrane. In the non-test mode, theinlet and the membrane are disconnected, typically by closing a valve.However, test gas trapped in the volume between the valve and themembrane continues to permeate through the membrane and is sensed by thetest gas sensing unit, despite the fact that the valve is closed.Permeation of the test gas through the membrane can continue forunacceptably long times, even for relatively small trapped volumes.Thus, there is need for leak detector configurations, which alleviatethis problem.

Prior art configurations have terminated transmission of the test gas bydeenergizing a membrane heater. However, it is desirable to very rapidlyterminate transmission of the test gas through the membrane inconfigurations where the membrane operates between ambient temperatureand about 100° C.

Preferably, transmission of the test gas through the membrane issubstantially terminated in a time of 500 milliseconds or less when theleak detector is switched from a test mode to a non-test mode or whentermination of test gas transmission through the membrane is requiredfor any other reason. More preferably, test gas transmission through themembrane is substantially terminated in a time of 100 milliseconds orless. Most preferably, test gas transmission through the membrane issubstantially terminated in a time of 25 milliseconds or less. As usedherein, substantial termination of test gas transmission through themembrane refers to reducing test gas transmission through the membraneto less than 10% of its initial value, i.e., reducing test gastransmission by more than 90%. These parameters permit multiple leaktests to be performed in rapid succession without interaction betweenleak tests and without buildup of excess quantities of the test gas inthe test gas sensing unit.

A schematic block diagram of a first example of a leak detector is shownin FIG. 1. A test port or leak detector inlet 10 is coupled through avalve 12 to a vacuum pump 14 and to an inlet of a valve, which can be asolenoid valve 20. As discussed below, solenoid valve 20 includes anintegral membrane 30. An outlet of the solenoid valve 20 is coupled to atest gas sensing unit 22, which provides an electrical signalrepresentative of test gas concentration to a signal processor 24.

Membrane 30 is incorporated into solenoid valve 20. The valve andmembrane configuration limits the trapped gas volume on the inlet sideof the valve when the valve is closed. The membrane 30 is a type, whichpermits operation between ambient temperature and about 100° C. Themembrane 30 is described in detail below.

In one embodiment, vacuum pump 14 is a mechanical vacuum pump. Test gassensing unit 22 can be an ion pump, an ionization gauge, a gas-consumingvacuum gauge, a mass spectrometer, a magnetic sector mass spectrometer,a quadrupole mass spectrometer, a penning cell, or a charge coupleddevice.

In operation, a test piece 26 or a sniffer probe (not shown) isconnected vacuum-tight to inlet 10. Then valve 12 is opened, thusconnecting inlet 10 to vacuum pump 14. Vacuum pump 14 reduces the inletpressure from atmospheric pressure to an operating or test pressurelevel. Valve 20 is then opened to perform a leak test.

In one operating mode, test piece 26 is connected to inlet 10 and a testgas, such as helium, is introduced onto the outer surface of the testpiece, such as by helium spray. Alternatively, if the test piece 26 ispressurized with helium, then the sniffer probe is moved around theoutside of the test piece. The helium is drawn through leaks into theinterior of the test piece 26, or is drawn into the sniffer probe, andpasses into the leak detector through inlet 10. The helium then passesthrough valves 12 and 20 and through membrane 30 to test gas sensingunit 22. The test gas sensing unit 22 detects and measures helium andprovides an output signal which is proportional to the quantity ofhelium in the gas sample received from the test piece or sniffer probe.The helium concentration is proportional to the leak rate of the testpiece.

A cross-sectional diagram of an example of solenoid valve 20 andmembrane 30 is shown in FIG. 2. Solenoid valve 20 includes a housing 40,which defines a valve inlet 42, a valve outlet 44 and a passage 46between inlet 42 and outlet 44. Membrane 30 is integrally mounted in andsealed to passage 46 so that gas that flows through the valve isrequired to pass through membrane 30. Solenoid valve 20 further includesa valve closure member 50 and a coil 52 for moving valve member 50between open and closed positions. As shown in FIG. 2, valve member 50includes a plunger 54 and a rigidly-connected tip element 56. When coil52 is energized, plunger 54 and tip element 56 are retracted to an openposition, as shown in FIG. 2, thereby allowing flow of the test gas frominlet 42 through membrane 30 to outlet 44. When coil 52 is de-energized,a spring 58 biases tip element 56 into contact with membrane 30 so as toclose the valve 20 and block the flow of gas through membrane 30. Tipelement 56 is configured so that it fully blocks passage 46 and is incontact with membrane 30 in the closed position of the valve.

The tip element 56 may include a thin layer of relatively low durometerelastomer on a surface 56 a that contacts membrane 30. When the solenoidvalve 20 is de-energized, spring 58 pushes tip element 56 againstmembrane 30. The compliant elastomer material seals across the entireface of membrane 30, squeezing essentially all gas from between thesurfaces. This configuration limits the trapped gas volume on membrane30 when solenoid valve 20 is closed, as the trapped gas can contain alarge number of test gas atoms. The valve stroke between open and closedpositions may be small to enable rapid opening and closing of the valve.

Valve 20 has been described as a solenoid valve having an integralmembrane, in which the valve closure member contacts the membrane in theclosed position of the valve. It will be understood that different valvetypes and configurations may be utilized to limit the trapped gas volumeon the inlet side of the membrane when the valve is closed.

A schematic block diagram of a second example of a leak detector isshown in FIG. 3. Like elements in FIGS. 1 and 3 have the same referencenumerals. Inlet 10 is coupled through valve 12 to a first inlet 70 of avacuum pump 72. Inlet 10 is also coupled through valve 12 and a valve 74to an inlet side of a membrane 80. Inlet side of membrane 80 isconnected through a valve 82 to a second inlet 84 of vacuum pump 72. Avalve 86 is connected between the inlet side of membrane 80 and anambient air inlet 88. As shown in FIG. 3, valves 12 and 74 and firstinlet 70 of vacuum pump 72 are connected to a node 90; and valves 74, 82and 86 and the inlet side of membrane 80 are connected to a node 92. Theconduits connected to node 92, i.e. the conduits that interconnectvalves 74, 82 and 86 and membrane 80, define a volume 96. A controller98 controls the opening and closing of valves 12, 74, 82 and 86.

A sensing side of membrane 80 is connected to test gas sensing unit 22,which provides an electrical signal representative of test gasconcentration to signal processor 24. In the example of FIG. 3, test gassensing unit 22 includes an ionization gauge 100, an ion pump 102 and agetter 104, all of which receive gas through membrane 80. In anotherexample, the test gas sensing unit includes a gas consuming vacuum gaugeand a getter.

The vacuum pump 72 may be a multi-stage diaphragm pump, a multi-stageroots pump, a multi-stage hook and claw pump, a multi-stage rotary orrolling piston pump, or a scroll pump. In one embodiment, vacuum pump 72is a 50-liter-per-minute scroll pump having two inlets. Inlet 84 has alower base pressure than inlet 70, so that the pressure in volume 96 isreduced when valve 82 is opened. In one example, inlets 84 and 70 mayhave base pressures (i.e., no mass flow condition) of about 100-300millitorr and 100-300 Torr, respectively. In another embodiment,separate vacuum pumps are utilized. In this embodiment, a first vacuumpump is coupled to an inlet side of valve 74 (node 90), and a secondvacuum pump is coupled to valve 82. As in the previous embodiment, thesecond vacuum pump has a lower base pressure than the first vacuum pump.

A small gas volume, typically less than 0.05 cubic centimeter, istrapped in volume 96 between valves 74, 82 and 86 and membrane 80.During a leak test, valve 12 is open, valve 86 is closed and valves 74and 82 are actuated sequentially. With valve 82 closed, valve 74 isopened for a time period on the order of 10 to 500 milliseconds toperform a leak test. Valve 74 is then closed. Valve 82 is openedimmediately to discharge volume 96 to inlet 84 of vacuum pump 72. Thisvery quickly reduces the total pressure and the test gas partialpressure at membrane 80 by a factor of about 200. The repeated cyclingof valves 74 and 82 results in a small mass flow rate to second inlet84. It will be understood that the above parameter values are given byway of example only and are not limiting as to the scope of theinvention.

Should the membrane 80 be exposed to high levels of the test gas, valve86 can be opened in order to purge volume 96 with ambient air. Theambient air has a low partial pressure of the test gas.

The test gas sensing unit 22 is sensitive to the test gas, in this casehelium. The ionization gauge 100 can sense extremely low partialpressures of helium. The ion pump 102 pumps away residual gases andhelium. The current profile of both the ionization gauge 100 and the ionpump 102 can be used to compute helium partial pressure. Getter 104 maybe utilized to capture hydrogen and minimize the false signal due tohydrogen interaction with the ionization gauge 100 or the ion pump 102.

In the leak detectors of FIGS. 1 and 3, membranes 30 and 80 are locatedbetween inlet 10 and test gas sensing unit 22. The membrane is amaterial that is permeable to the test gas used in the leak detector.The test gas is a light gas, such as helium, hydrogen, neon, andisotopes thereof. Helium is commonly used as the test gas. The membranesubstantially passes the test gas while substantially blocking heaviergases, liquids and particles. The membrane thus acts as a test gaswindow by allowing the test gas to pass while blocking heavier gases,liquids and particles. In particular, the membrane has a relatively highpermeance to the light test gas and a relatively low permeance toheavier gases, i.e., all gases heavier than neon. Thus, inlet 10 andtest gas sensing unit 22 can be at different pressures. The membrane canbe any suitable material that is permeable to the test gas and can haveany shape or dimension. Examples of suitable materials include, but arenot limited to, quartz glass, or silica, and permeable polymers such astetrafluoroethylene. In the leak detectors of FIGS. 1 and 3, themembrane is operated between ambient temperature and about 100° C.

Preferably, the membrane is a composite membrane that includes a porousmembrane, which may function as a substrate, and a semi-permeablemembrane in series. The net permeance of the composite membrane issubstantially constant and independent of temperature in the vicinity ofa design temperature T₀.

A semi-permeable membrane transmits only certain gases. Examples ofsemi-permeable membranes include variations of silicon dioxide,including quartz, quartz glass, fused silica, silica, Pyrex, etc. Othersemi-permeable membranes include polymers such as: Teflon, Tefzel, FEP,Kapton, Mylar, acetate, polyamide, etc. Light gases permeate throughthese membranes at significant permeation rates, but heavier gases, suchas nitrogen and oxygen, have permeation rates many orders of magnitudelower. Light gases diffuse through a semi-permeable membrane by solidstate diffusion. The pore size of the semi-permeable membrane iscomparable to the gas atomic diameter, and the atoms move within themembrane lattice due to random thermal motion. The transmission rate perunit area, per unit pressure difference across the semi-permeablemembrane of a particular gas, the permeance having units of [mole/(m²second Pascal)], is a strong function of temperature. The permeance ofthe semi-permeable membrane increases with increasing temperature.

The prior art has utilized the temperature dependence of quartzpermeability to control helium transmission to a leak detector. A quartzdisk or tube is typically heated to 400° C.-750° C. to obtain high ratesof helium diffusion through the quartz. Heavy gases such as air areexcluded because their atomic diameters are too large for almost allpores. Under certain operating conditions, such as the presence of toomuch helium, or in standby mode, the quartz temperature is reduced byswitching off the heater. This reduces the permeation rate by a factorof 100-300. For these devices, a membrane permeance that is a strongfunction of temperature is necessary and desirable.

A porous membrane transmits substantially all gases to varying degrees,depending on pore geometry. Pore diameters are several to many times thegas atomic diameters. Examples of porous membranes include sinteredmetals, alumina, zeolites, silicon carbide, etc., and certain glassessuch as Corning Vycor 7930 glass. The primary method of gas transportthrough a porous membrane is by Knudsen diffusion. In Knudsen diffusion,the mean free path of a gas atom is much greater than the characteristicpore diameter. This results in a permeance that varies as T^(−1/2)Permeance in Knudsen diffusion decreases with increasing temperature.

In general, the permeance of a porous membrane is orders of magnitudegreater than the permeance of a practical semi-permeable membrane. Ifthe porous membrane and the semi-permeable membrane are placed in series(so that the gas passes through both the porous membrane and thesemi-permeable membrane) to form a composite membrane, the low permeanceof the semi-permeable membrane dominates and the net permeance remains astrong function of temperature. If the permeances of the porous andsemi-permeable membranes are of the same order of magnitude, then one ormore design parameters can potentially be altered so that localtemperature independence at a design temperature T₀, typically thenominal operating temperature, is achieved.

Preferably, the net permeance of the composite membrane has a permeanceto the test gas that varies by less than five percent over a temperaturerange of T₀−20K to T₀+20K, where T₀ is the design temperature and K isdegrees Kelvin. More preferably, the membrane has a net permeance to thetest gas that varies by less than one percent over the temperature rangeof T₀−20K to T₀+20K. The design temperature T₀ may be about 300K. Itwill be understood that different variations in permeance and differenttemperature ranges are included within the scope of the presentinvention. For example, a membrane having a permeance to the test gasthat varies by less than five percent over the operating temperaturerange of the leak detector may be utilized.

A design method is now described. A functional approximation ofpermeance as a function of temperature for the porous membrane and thesemi-permeable membrane is needed. Since the two permeances are inseries, the resulting net permeance is

$\begin{matrix}{\frac{1}{N(T)} = {\frac{1}{P(T)} + \frac{1}{S(T)}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where N=net permeance, P=porous permeance, and S=semi-permeablepermeance. Permeances P and S are a function of temperature T and ofvarious geometric and physical constants, some of which can be changed.Equation (1) is solved for permeance N and then is differentiated withrespect to temperature T. When

$\begin{matrix}{\frac{N}{T} = {{0\mspace{14mu} {at}\mspace{14mu} T} = T_{0}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

is satisfied, N(T) is approximately constant in the vicinity oftemperature T₀. The resulting equation to be satisfied is

$\begin{matrix}{{{P^{2}\frac{S}{T}} + {S^{2}\frac{P}{T}}} = {{0\mspace{14mu} {evaluated}\mspace{14mu} {at}\mspace{14mu} T} = T_{0}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Since dP/dT is less than zero and all other terms are positive, asolution is possible, but not guaranteed. Material properties anddimensions which result in permeances P(T) and S(T) may not allow asolution. It has been determined that when a thin silica layer iscombined with a moderate thickness of Vycor 7930 glass, a family ofsolutions is possible.

Preferred embodiments include a substrate of Vycor 7930 glass of aselected thickness with a thin layer of silica deposited on one or bothsides of the glass substrate using chemical vapor disposition oftetraethyl orthosilicate (TEOS). Process details are described in U.S.Pat. No. 6,527,833, issued Mar. 4, 2003 to Oyama et al., which is herebyincorporated by reference. Corning Vycor 7930 glass is a porous glasswith a very uniform pore diameter of approximately 3.6 nanometers. Athin silica layer (approximately 10 nanometers) results in a permeancegreater than that of the Vycor glass.

The design method described above results in a solution with a Vycorglass thickness of 1.45 millimeters and a silica thickness of 10nanometers. The permeance of the composite membrane has zero temperaturedependence at T=300K and only very small deviations in the vicinityaround 300K, as shown in FIG. 4. In FIG. 4, curve 120 represents thepermeance of a silica layer as a function of temperature, curve 122represents the permeance of Vycor glass as a function of temperature,and curve 124 represents the net permeance of a composite membrane ofsilica and Vycor glass as a function of temperature. Deviations from the300K permeance value are less than 0.1 percent from 280K to 320K. Thistranslates to a temperature coefficient of approximately 0.005% perdegree C. By comparison, the permeance of a quartz layer by itself has atemperature coefficient of more than 2% per degree C. at 300K.

A composite membrane having flat geometry such as a disk may bedesirable. In this case, it may be easier to coat the entire disk withsilica. This results in two silica layers of equal thickness on oppositesides of a Vycor glass disk or sheet. The design process is similar tothat described above. For a silica layer of equal thickness on oppositesides of a porous material,

$\begin{matrix}{{{2P^{2}\frac{S}{T}} + {S^{2}\frac{P}{T}}} = {{0\mspace{14mu} {evaluated}\mspace{14mu} {at}\mspace{14mu} T} = T_{0}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

where P=porous permeance, S=single layer permeance, and

$\begin{matrix}{{N(T)} = \frac{SP}{{2P} + S}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

The deposition process may be performed in a helium environment ratherthan in argon. The advantage of using helium is that once both sides ofthe Vycor glass are sealed, helium can diffuse out, whereas argon wouldbe trapped. When the Vycor glass is sealed with silica, water andhydrocarbon contaminants cannot enter. This simplifies storage of thecomposite membrane.

FIG. 5 shows the permeances of a Vycor glass substrate having athickness of 2.327 millimeters and an 8 nanometer layer of silica onboth sides. In FIG. 5, curve 130 represents the permeance of silica,curve 132 represents the permeance of Vycor glass, and curve 134represents the net permeance of a composite membrane of Vycor glass witha silica layer on both sides. Deviations from the 300K permeance valueare less than 0.1 percent from 280K to 320K.

For the composite membrane described above, the variables available tosatisfy equation (3) include silica layer thickness, Vycor glassthickness and Vycor glass pore diameter. The thickness of the silicalayer is generally linear with TEOS decomposition time. Thicker orslightly thinner layers of silica can be deposited. Porous glasses withdifferent pore diameters can be obtained. Various combinations of thethree variables enable flexibility in satisfying equations (3) and (4)and in designing for a specific net permeance.

A reference leak unit 180 is shown in FIG. 6. A reference leak may beused to calibrate a leak detector. Reference leak unit 180 may include acontainer 182 having an opening that is sealed by a membrane 184.Preferably, membrane 184 is a composite membrane as described above. Aninterior volume 186 of the reference leak unit contains a test gas, suchas helium. The helium leaks from the container 182 at a known rate thatdepends on the permeance of membrane 184.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A leak detector comprising: a leak detector inlet to receive a testgas; a vacuum pump coupled to the leak detector inlet; a test gassensing unit connected through a passage to the leak detector inlet tosense the test gas; a membrane that is permeable to the test gas, themembrane being disposed in the passage between the leak detector inletand the test gas sensing unit; and a control mechanism to substantiallyterminate transmission of the test gas through the membrane in a time of500 milliseconds or less.
 2. The leak detector as defined in claim 1,wherein the membrane has an operating temperature from ambienttemperature to about 100° C.
 3. The leak detector as defined in claim 1,wherein the control mechanism comprises: a first valve coupled betweenthe leak detector inlet and the membrane, a trapped volume being definedbetween the first valve and the membrane; a second valve coupled to thetrapped volume; and a controller to open the first valve and close thesecond valve in a first mode and to open the second valve and close thefirst valve upon switching to a second mode, wherein the vacuum pumpcomprises a dual inlet vacuum pump having a first inlet coupled to theleak detector inlet and a second inlet coupled through the second valveto the trapped volume.
 4. The leak detector as defined in claim 1,wherein the control mechanism comprises a valve disposed between theleak detector inlet and the test gas sensing unit, the valve including ahousing and a valve closure member movable between a closed position incontact with the membrane and an open position retracted from themembrane.
 5. The leak detector as defined in claim 4, wherein the valvecomprises a solenoid valve.
 6. The leak detector as defined in claim 4,wherein the membrane is incorporated into the valve housing.
 7. The leakdetector as defined in claim 4, wherein the valve member includes anelastomer portion that contacts the membrane.
 8. The leak detector asdefined in claim 1, wherein the control mechanism is configured tosubstantially terminate transmission of the test gas through themembrane in a time of 100 milliseconds or less.
 9. The leak detector asdefined in claim 1, wherein the control mechanism is configured tosubstantially terminate transmission of the test gas through themembrane in a time of 25 milliseconds or less.
 10. The leak detector asdefined in claim 1, wherein the membrane has a permeance to the test gasthat varies by less than five percent over a temperature range of T₀−20Kto T₀+20K, where T₀ is a nominal operating temperature.
 11. The leakdetector as defined in claim 10, wherein the nominal operatingtemperature T₀ is 300K.
 12. A method for leak detection, comprising:passing a test gas through a membrane disposed in a passage between aleak detector inlet and a test gas sensing unit, and sensing the testgas with the test gas sensing unit; and substantially terminatingtransmission of the test gas through the membrane in a time of 500milliseconds or less.
 13. The method for leak detection as defined inclaim 12, wherein substantially terminating transmission of the test gasthrough the membrane comprises rapidly vacuum pumping a trapped volumeon an inlet side of the membrane upon switching from a first mode to asecond mode.
 14. The method for leak detection as define in claim 12,wherein substantially terminating transmission of the test gas throughthe membrane comprises moving a valve closure member into contact withthe membrane upon switching from a first mode to a second mode.
 15. Themethod for leak detection as defined in claim 12, further comprisingoperating the membrane at a temperature from ambient temperature toabout 100° C.
 16. The method for leak detection as defined in claim 11,wherein substantially terminating transmission of the test gas throughthe membrane comprises substantially terminating transmission of thetest gas through the membrane in a time of 100 milliseconds or less. 17.The method for leak detection as defined in claim 11, whereinsubstantially terminating transmission of the test gas through themembrane comprises substantially terminating transmission of the testgas through the membrane in a time of 25 milliseconds or less.